1. Field of the Invention
The present invention relates to a method of manufacturing a wavelength conversion device which make use of a coherent light source for use in the industrial field of optical data storage, optical information processing and applied optic measurement control.
2. Description of Prior art
Domain inversion for forcibly inverting a domain in a ferroelectric is employed in optical frequency modulators which utilize surface acoustic waves and wavelength conversion devices which utilize the inversion of a non-linear domain in such a manner periodical domain-inverted structures are formed in a ferroelectric. In particular, by periodically inverting the non-linear domain of non-linear optical materials, a second harmonic generating device exhibiting an extremely high conversion efficiency can be manufactured. By using the second harmonic generating device thus-manufactured to convert a beam such as semiconductor laser, a small-size short wavelength light source can be realized which can be widely applied to the printing field, the optical information processing field, the applied optic measurement and control field and the like. Therefore, a large number of studies have been made about the domain inversion.
FIG. 21 is a structural view of a conventional light wavelength conversion device. Then, a detailed description will be made about generation of higher harmonic waves (wavelength of which is 0.41 .mu.m) with respect to the fundamental waves the wavelength of which is 0.82 .mu.m (refer to E. J. Lim, M. M. Fejer, R. L. Byer, and W. J. Kozlovsky, "Blue light generation by frequency doubling in periodically poled lithum niobate channel waveguide", Electron. Lett., 25,731-732, (1989)).
As shown in FIG. 21, an optical waveguide 44 is formed in an LiNbO.sub.3 substrate 41 and a layer (a domain-inverted structure) 45 is formed in the optical waveguide 44, the domain of the layer 45 being periodically inverted. By compensating inconsistence in propagation coefficient between the fundamental waves and the higher harmonic wave with the periodic structure of the domain-inverted structure 45, the higher harmonic waves can be efficiently generated. When fundamental waves P1 (43) are made to be incident upon the entrance surface of the optical waveguide 44, higher harmonic waves P2 (42) are efficiently generated from the optical waveguide 44 so that the above-described structure is able to act as an optical wavelength conversion device.
An optical wavelength conversion device of the conventional type described above has been basically formed into a domain-inverted structure. Then, a method of manufacturing a device of the type described above will now be described with reference to FIGS. 22a to 22c. Referring to FIG. 22a, a Ti pattern 101 is formed on an LiNbO.sub.3 substrate 100 composed of non-linear optical crystal at widthwise intervals of several milimeters by lifting-off and evaporation. Then, heat treatment at about 1100.degree. C. is performed in a state shown in FIG. 22b so that a domain-inverted structure 102, the direction of the domain of the LiNbO.sub.3 substrate 100 of which is inverted, is formed. Then, heat treatment is performed in benzoic acid (200.degree. C.) for 20 minutes in a state shown in FIG. 22c, and then annealing is performed at 350.degree. C. so that an optical waveguide 103 is formed. As a result, the optical wavelength conversion device manufactured by using the above-described benzoic acid treatment can produce higher harmonic waves P2, the power of which was 940 nW, with respect to fundamental wave P1, the wavelength of which was 0.82 mm, under conditions that the length of the optical waveguide was 1 mm and the power of the fundamental wave P1 was made to be 14.7 mW. At this time, a conversion efficiency of 0.43%/W was obtained. When 1 W of fundamental waves are made incident, higher harmonic waves of 370 mW can be delivered in a case where the length of the device is 10 mm. In this case, a device, the length of which is 10 mm, exhibits a conversion efficiency of 37%/W.cm.sup.2 per W.
There has been disclosed a report in Appl. Phys. Lett. by Kiyoshi Nakamura, 1990, Vol. 56, p.p 1535 in which a fact was reported that a domain-inverted structure could be formed in an LiTaO.sub.3 member. Referring to FIGS. 23a to 23b, reference numeral 104 represents an LiTaO.sub.3 substrate, 105 represents a proton-exchange layer and 106 represents a domain-inverted structure. As shown in FIG. 23a, the domain-inverted structure is manufactured in such a manner that the LiTaO.sub.3 substrate 104 is subject to heat treatment at 590.degree. C. in benzoic acid so that the proton-exchange layer 105 is formed. Then, the LiTaO.sub.3 substrate 104 is, as shown in FIG. 23c, subjected to heat treatment at 570.degree. C. to 590.degree. C. in the vicinity of the Curie point. As a result, the domain-inverted structure 106 is formed on the -C surface of the LiTaO.sub.3 substrate 104. However, there has not been an idea of using the above-described structure for the wavelength conversion device.
The LiNbO.sub.3 crystals involve problems of an optical damage. Also, variation in the refractive index of the waveguide due to a rise in the power density of light cannot be prevented, causing the phasematch condition to vary. Therefore, there is raised a problem in that a waveguide conversion device capable of stably operating and exhibiting excellent conversion efficiency can hardly be manufactured. Accordingly, studies have been made for the purpose of manufacturing the wavelength conversion device by forming a domain-inverted structure in LiTaO.sub.3 crystal which are able to resist against the optical damage. Furthermore, the LiTaO.sub.3 crystal exhibits excellent optical characteristics and the mixture of impurities can be satisfactorily prevented at the time of forming the crystal. Therefore, since the crystallization characteristics LiTaO.sub.3 is superior to LiNbO.sub.3, excellent optical damage resistance and DC drift resistance can be obtained and thereby it can be utilized as an advantageous optical material for an optical IC device. However, there arises a problem in that, although a slab-shaped inverted structure can be formed by the above-described method, a desired periodical domain-inverted structure cannot be formed in the LiTaO.sub.3 crystal.